When designing printheads containing a plurality of ink-ejecting nozzles in a densely packed array, it is necessary to provide some means of isolating the dynamics of any given nozzle from its neighbors, or else cross-talk will occur between the nozzles as they fire droplets of ink from elements associated with the nozzles. This cross-talk seriously degrades print quality and hence any providently designed ink-jet printhead must include some features to accomplish decoupling between the nozzles and the common ink supply plenum so that the plenum does not supply a cross-talk path between neighboring nozzles.
Further, when an ink-jet printhead is called upon to discharge ink droplets at a very high rate, the motion of the meniscus present in each nozzle must be carefully controlled so as to prevent any oscillation or "ringing" of the meniscus caused by refill dynamics from interfering with the ejection of subsequently fired droplets. Ordinarily, the "setting time" required between firing sets a limit on the maximum repetition rate at which the nozzle can operate. If an ink droplet is fired from a nozzle too soon after the previous firing, the ringing of the meniscus modulates the quantity of ink in the second droplet out. In the case where the meniscus has "overshot" its equilibrium position, a firing superimposed on overshoot yields an unacceptably large ejected droplet. The opposite is true if the firing is superimposed on an undershoot condition: the ejected droplet is too small. Therefore, in order to enhance the maximum printing rate of an ink-jet printhead, it is necessary to include in its design some means for reducing meniscus oscillation so as to minimize the settling time between sequential firings of any one nozzle.
Previous approaches to the problem of cross-talk, or minimizing inter-nozzle coupling, can be separated into three classes: resistive, inertial, and capacitive. The following is a brief discussion of each method and a critique of the typical embodiments of these methods.
Resistive decoupling uses the fluid friction present in the ink feed channels as a means of dissipating the energy content of the cross-talk surges, thereby preventing the dynamics of any single meniscus from being strongly felt by its nearest neighbors. In the prior art, this is typically implemented by making the ink feed channels longer or smaller in cross-section than the main supply plenum. While these are simple solutions, they have several drawbacks. First, such solutions rely upon fluid motion to generate the pressure drops associated with the energy dissipation; as such, they can only attenuate the cross-talk surges, not completely block them. Thus, some cross-talk "leakages" will always be present. Second, any attempt to shut off cross-talk completely by these methods will necessarily restrict the refill rate of the nozzles, thereby compromising the maximum rate at which the printhead can print. Third, the resistive decoupling techniques as practiced in the prior art add to the inertia of the fluid refill channel, which has serious implications for the printhead performance (as will be explained at the end of the inertial decoupling exposition which follows shortly).
In capacitive decoupling, an extra hole is put in the nozzle plate above that point where the ink feed channel meets the ink supply plenum. Any pressure surges in the ink feed channel are transformed into displacements of the meniscus present in the extra hole (or "dummy nozzle"). In this way, the hole acts as an isolator for brief pressure pulses but does not interfere with refill flow. The location, size and shape of the isolator hole must be carefully chosen to derive the required degree of decoupling without allowing the hole to eject droplets of ink as if it were a nozzle. This method is extremely effective in preventing cross-talk (but can introduce problems with nozzle meniscus dynamics, as will be discussed below).
In inertial decoupling, the feed channels are made as long and slender as possible, thereby maximizing the inertial aspect of the fluid entrained within them. The inertia of the fluid "clamps" its ability to respond to cross-talk surges in proportion to the suddenness of the surge and thereby inhibits the transmission of cross-talk pulses into or out of the ink feed channel. While this decoupling scheme is used in the prior art, it requires considerable area ("real estate") within the print head to implement, making a compact structure impossible. Furthermore, since the resistive component of a pipe having a rectangular cross-section scales directly with length and inversely with the third power of the smaller of the two cross-section dimensions, the flow resistance can grow to an unacceptable level, compromising refill speed. More importantly, however, are the dynamic effects caused by the coupling of this inertance to the compliance of the nozzle meniscus, as will be discussed below.
With regard to the problem of meniscus dynamics, there are apparently no solutions offered in the prior art. Apparently, this is a problem that has only recently surfaced as printhead designs have been pushed to accommodate higher and higher repetition rates. Clearly, any method used to decouple the dynamics of neighboring nozzles will also aid in damping out meniscus oscillations, at least from a superficial consideration. In practice, problems are experienced when trying to use the decoupling means as the oscillatory damping means. These problems can be traced to the synergistic effects between the nozzle meniscus and the fluid entrained within the ink feed channel, as outlined below.
If resistive decoupling is attempted by reducing the width of the entire ink feed channel, the inertia of the fluid entrained within the feed channel increases. When this inertia is coupled to the compliance of the meniscus in the nozzle, it results in a lower resonant frequency of oscillation of the meniscus, which requires a longer settling time between firings of the nozzle. The inertial effect and the resistive effect are hence deadlocked, with the net effect being that settling time cannot be reduced.
Capacitive decoupling has been proven effective at droplet ejection frequencies below that corresponding to the resonant frequency of the nozzle meniscus coupled to the feed channel inertia. However, its implementation at frequencies near meniscus resonance is also complicated by interactive effects. Specifically, the isolator orifice acts as a low impedance shunt path for high frequency surges. Hence, the high frequency impedance of an ink feed channel terminated at its plenum end with an isolator orifice will be lower than an equivalent channel without an isolator. This means that during the bubble growth phase, blow-back flow away from the nozzle is increased by the isolator orifice. This robs kinetic energy from the droplet emerging from the nozzle, which results in smaller droplet size and lower droplet velocities and thus lower ejection efficiency. During the bubble collapse phase, the isolator orifice meniscus pumps fluid flow back into the refill chamber, which excites a resonant mode in which the two menisci trade fluid between themselves via the ink feed channel. Since these two menisci are for most practical designs similar in size, and since they are effectively "in series", the equivalent compliance of the coupled system is roughly half of that with only one orifice in it. The two-orifice system will thus resonate at a higher frequency, which is a benefit from a settling time point of view, but the energy stored in the resonating system still needs to be dissipated and therefore constrictive damping will be necessary in such an implementation. While the effects of these resonances is poorly understood at this time, the efficiency decrease may be severe enough to prevent the printhead from working.
It is clear that what is needed is a printhead structure that accomplishes both (1) isolation of any given nozzle from its neighbors and (2) reduced oscillation of the meniscus caused by refill dynamics from interfering with the ejection of subsequently fired droplets, while limiting the severity of any side effects incurred in the implementation of the desired structure.